In the CDMA mode, a plurality of mobile stations share a same frequency band to communicate with a base station. Therefore, in the case of a communication between a mobile station A and a base station, for example, a signal from another mobile station B to the base station (an undesired signal) interferes a signal sent from the mobile station A to the base station (a desired signal) to obstruct a communication between the mobile station A and the base station. Similarly, the signal sent from the mobile station A obstructs a communication between the mobile station B and the base station.
The interference level increases relative to a receiving level of an undesired signal wave received by the base station. The receiving level of the undesired signal is relative to the transmit power at the time the undesired signal is sent from a mobile station. Thus, it is necessary for the base station to control the transmit power from mobile stations so that the receiving level in the base station should be always minimum essential, in order to keep the interference level minimum. Under an ideal situation of this control, the number of channels available for communication becomes the largest, while the number of channels available for a communication would decrease as the situation goes away from the ideal.
Concerning a technology in controlling transmit power in a CDMA mobile communication, provided a transmit power controlling method described in IMT-2000 Study Committee, Air-interface WG, SWG Document Title: Volume 3 Specifications of Air-Interface for 3G Mobile System, Source: SWG, Version: 0-4.0, Date: Dec. 18, 1997 issued by DENPA SANGYOUKAI, for example (referred to as “W-CDMA mode”, hereinafter). A transmit power controlling method in the W-CDMA mode will be described below. In the description, an upstream direction means a direction of sending signals from a mobile station to a base station and a downstream direction means a direction of sending signals from a base station to a mobile station.
A base station measures a signal to interference power ratio (SIR) of a signal sent from a mobile station in the upstream direction to transmit a transmit power controlling signal in accordance with the measured SIR. A structural diagram of a conventional base station is shown in FIG. 29. A signal received by an antenna 210 passes through a circulator 211 to take a process of modulating a base band signal and a process of receiving at a high/middle frequency in a radio module for receiving 212. The base station carries out processes of synchronous capturing and reverse spreading of the received signal in synchronous capturing/reverse spreading circuits 213a to 213n in which a parameter is set for each mobile station, since the received signal is a multiplex signal from a plurality of mobile stations (referred to as MSa to MSn). The signal output from the capturing/reverse spreading circuits 213a to 213n is input to detecting potions 214a to 214n, respectively, to take a detecting process such as compensation for a phase rotation. The signal output from the detecting portions 214a to 214n is input to demodulating portions 215a to 215n, respectively, to take an error controlling process such as de-interleave and Viterbi decoding, and then, are used as received data.
On the other hand, the signal output from the capturing/reverse spreading circuits 213a to 213n is input to an upstream channel SIR measuring portion 221 through signal lines 220a to 220n, respectively. The upstream channel SIR measuring portion 221 measures SIR of the received signal input through the signal lines 220a to 220n, respectively (referred to as SIRa to SIRn), to input the SIRa to SIRn to an upstream channel transmit power controlling signal generating portion 222 through the signal lines 230a to 230n. 
The upstream channel transmit power controlling signal generating portion 222 compares SIRa to SIRn with target SIRs given for MSa to MSn in advance by a controlling portion 500 (referred to as T-SIRa to T-SIRn) to generate transmit power controlling signals (TPCa to TPCn) for MSa to MSn. The controlling portion 500 is an element that controls a whole base station and that transmits various signals to each element in the base station. In FIG. 29, signal lines are omitted other than the line to the upstream channel transmit power controlling signal generating portion 222 in order to simplify the diagram. A structure of the controlling portion/the upstream channel transmit power controlling signal generating portion 222 is shown in FIG. 30. The upstream channel transmit power controlling signal generating portion 222 comprises transmit power controlling signal generating portions 222a to 222n whose inputs are SIRi and T-SIRi and whose outputs are TPCi. The added character “i” denotes one of characters “a” to “n”. A structure of a transmit power controlling signal generating portion 222i is shown in FIG. 31. A comparator 223i compares SIRi and T-SIRi input through a signal line 230i to generate a signal that selects 0 in a selector 224i in the case of SIRi≧T-SIRi, and a signal that selects 1 in the selector 224i in the case of SIRi<T-SIRi. The selector 224i selects either 0 or 1 in accordance with an output from the comparator 223i to output it as TPCi through a signal line 231i. The TPCi=0 is a signal that instructs a mobile station to reduce the transmit power. On the contrary, TPCi=1 is a signal that instructs a mobile station to increase the transmit power.
Frame forming portions 225a to 225n shown in FIG. 29 form transmission data to MSa to Msn, the data which took an error controlling process such as fold-encoding and interleave in encoding portions 222a to 222n, and the transmit power controlling signals TPCa to TPCn, which are input from the upstream channel transmit power controlling signal generating portion 222, into frames in accordance with a format defined in the system. Spreading circuits 223a to 223n carry out a spectrum spreading process for outputs from the frame forming portions with a parameter corresponding to MSa to MSn. An adding circuit 226 adds transmission signals in order to multiplex signals for MSa to MSn. The transmission signals output from the adding circuit 226 are transmitted from the antenna 210 after passing through a radio module for transmission 224 and a circulator 211.
A mobile station MSi receives the aforementioned transmit power controlling signal TPCi to change the transmit power in accordance with a result of demodulation. A structure of a conventional mobile station is shown in FIG. 32. A signal received by the antenna 10 passes through a circulator 11 to take a process of demodulating a base band signal and a process of receiving at a high/middle frequency in a radio module for receiving 12.
A mobile station carries out processes of synchronous capturing and spectrum reverse spreading of the received signal in a synchronous capturing/reverse spreading circuit 13 in which a parameter is set for a channel being used in the mobile station, since the received signal is a multiplex signal in a plurality of channels. The signal output from the synchronous capturing/reverse spreading circuit 13 takes a detecting process such as compensation for a phase rotation in a detecting portion 14 and takes an error controlling process such as de-interleave and Viterbi decoding in demodulating portion 15, so as to be used as received data.
After output from the detecting portion 14, the received transmit power controlling signal passes through a signal line 16 and is input to a transmit power controlling signal determining portion 40. The transmit power controlling signal determining portion 40 determines whether the received transmit power controlling signal is “0” or “1”. The transmit power controlling signal determining portion 40 generates a controlling signal, which selects “−1 dB”, for example, as an output for a selector 41 when the result of determining the transmit power controlling signal is “0”, and generates a controlling signal, which selects “+1 dB”, for example, as an output for the selector 41 when the result of determining is “1”, to send the controlling signal to the selector 41.
The selector 41 outputs either “+1 dB” or “−1 dB”, for example, as a variation amount of the transmit power in accordance with a controlling signal input from the transmit power controlling signal determining portion 40.
A transmit power calculating portion 19 determines the changed transmit power, using the variation amount of the transmit power input from the selector 41 and the current transmit power input from a transmit power maintaining circuit 20. That is, the changed transmit power is increased by 1 dB from the current transmit power when the selector inputs “+1 dB”, and it is decreased by 1 dB from the current transmit power when the selector contrary inputs “−1 dB”.
The transmission signal takes an error controlling process such as, for example, fold-encoding and interleave, in the encoding portion 22 to form a frame of a format defined in the system in a frame forming portion 25, and then, takes a spectrum spreading process in a spreading circuit 23. A variable gain amplifier 21 amplifies transmission signals at a proper gain so that the signals could be transmitted at the designated transmit power from the transmit power calculating portion 19. The transmission signal output from the variable gain amplifier 21 passes through the radio module for transmission 24 and the circulator 11, and then, is transmitted from the antenna 10.
An example of a change of the transmit power of a mobile station in the case that the mobile station performs the above operation is shown as a solid line 62 in FIG. 33. The horizontal axis 60 shows the time, while the vertical axis 61 shows the transmit power of the mobile station. Results 83a to 83e of determining the transmit power controlling signal received at the times 120 to 124 are also shown in the horizontal axis 60. As shown in FIG. 33, the mobile station operates so as to increase the transmit power by 1 dB at the times 122 and 124 when the result of determining the controlling signal is “1” and so as to decrease the transmit power by 1 dB at the times 120, 121 and 123 when the result of determining the controlling signal is “0”.
There are two problems solved by the invention as described below.
First, there is a large possibility that the received transmit power controlling signal would result in being demodulated with an error, when the quality of receiving the transmit power controlling signal in a mobile station is bad. In this case, the conventional method that the demodulated result is determined either “0” or “1” increases the possibility that the transmit power controlling signal is incorrectly determined to be a value different from the proper value.
In the case that the transmit power controlling signal, which should be determined to be “1”, is incorrectly determined to be “0”, namely, in the case that a mobile station incorrectly decreases the transmit power at the time when the transmit power should be increased, the quality of a signal received from the above mobile station would deteriorate in the base station. This causes the quality of a communication to be deteriorated, and furthermore, causes a communication to be cut off.
To the contrary, in the case that the transmit power controlling signal, which should be determined to be “0”, is incorrectly determined to be “1”, namely, in the case that a mobile station incorrectly increases the transmit power at the time when the transmit power should be decreased, the interference amount by the above mobile station to another mobile station would increase in the base station. Therefore, deterioration of the communication quality and cut-off of a communication occur in the other mobile station. This means, at the same time, that the number of mobile stations available for a communication would decrease, and as a result, the communication capacity of the whole system will be reduced.
Moreover, the aforementioned deterioration of the communication quality and reduction of the communication capacity of the system appear more significantly, when the result of determining the transmit power controlling signal, whose receiving quality is bad, is biased to either “0” or “1” due to such as a component of direct current offset contained in a receiver in a mobile station.
Secondly, when a receiving operation stops earlier than a transmitting operation as a controlling sequence in the base station upon cutting off a communication, that is, an operation of the synchronous capturing/reverse spreading circuit 213i (i=1, 2, . . . , n) stops earlier than that of the upstream channel SIR measuring portion 221 in FIG. 29, it is considered that the operation of the upstream channel SIR measuring portion 221 would become unstable, since the upstream channel SIR measuring portion 221 tries to get SIRi from the output of the synchronous capturing/reverse spreading circuit 213i whose operation has stopped. In this case, an improper transmit power controlling signal TPCi is generated to be transmitted from the base station to the mobile station MSi. The mobile station MSi may perform transmission with an excessive transmit power, as a result of controlling the transmit power in accordance with the above-mentioned improper transmit power controlling signal TPCi. In this case, the interference amount by a signal of the mobile station MSi to another mobile station increases in the base station. Thus, the deterioration of the communication quality and cut-off of a communication occur in the other mobile station. This reduces the communication capacity of the whole system, like the first problem.